Biomimetic pathways for assembling inorganic thin films and oriented mesoscopic silicate patterns through guided growth

ABSTRACT

A process directed to preparing surfactant-polycrystalline inorganic nanostructured materials having designed microscopic patterns. The process includes forming a polycrystalline inorganic substrate having a flat surface and placing in contact with the flat surface of the substrate a surface having a predetermined microscopic pattern. An acidified aqueous reacting solution is then placed in contact with an edge of the surface having the predetermined microscopic pattern. The solution wicks into the microscopic pattern by capillary action. The reacting solution has an effective amount of a silica source and an effective amount of a surfactant to produce a mesoscopic silica film upon contact of the reacting solution with the flat surface of the polycrystalline inorganic substrate and absorption of the surfactant into the surface. Subsequently an electric field is applied tangentially directed to the surface within the microscopic pattern. The electric field is sufficient to cause electro-osmotic fluid motion and enhanced rates of fossilization by localized Joule heating.

This application is a continuation of Ser. No. 08/964,876 filed on Nov.5, 1997.

GOVERNMENT RIGHTS

The present invention has been made under a contract by the UnitedStates Army Research Office and the MRSEC program of the NSF and thegovernment may have certain rights to the subject invention.

SPECIFICATION BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a process for preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns using a polycrystalline inorganicsubstrate. More specifically, this invention relates to biomimeticallyassembling inorganic thin films, and to the synthesis of mesostructuredfilm using a supramolecular assembly of surfactant molecules atinterfaces to template the condensation of an inorganic silica lattice.Additionally, this invention relates to forming an ordered silicatestructure within a highly confined space.

2. Related Art

Biologically produced inorganic-organic composites such as bone, teeth,diatoms, and sea shells are fabricated through highly coupled (and oftenconcurrent) synthesis and assembly. These structures are formed throughtemplate-assisted self-assembly, in which self-assembled organicmaterial (such as proteins, or lipids, or both) form the structuralscaffolding for the deposition of inorganic material. They arehierarchically structured composites in which soft organic materials areorganized on length scales of 1 to 100 nm and used as frameworks forspecifically oriented and shaped inorganic crystals (that is, ceramicssuch as hydroxyapatite, CaCo₃, SiO₂, and Fe3O₄). In some cases,structurally organized organic surfaces catalytically or epitaxiallyinduce growth of specifically oriented inorganic thin films.

Most importantly, however, nature's way of mineralization usesenvironmentally balanced aqueous solution chemistries at temperaturesbelow 100° C. This approach provides an attractive alternative to theprocessing of inorganic thin films, especially in applications wheresubstrates cannot be exposed to high temperatures, or more generally inthe pursuit of increased energy efficiency.

Potential applications for dense, polycrystalline inorganic films span abroad range of industries. These include the possibility of applyinghard optical coatings to plastics in order to replace glass,abrasion-resistant coatings for plastic and metal components subject towear, and the deposition of oriented films of iron oxide phases for useas magnetic storage media. For many of these applications, conventionalceramic processing methods, which require high temperature sintering,cannot be used because of problems with substrate degradation.

A classic and a widely studied example of a biocomposite is the nacre ofabalone shell, in which thin films of organic (<10 nm) and inorganic(<0.5 μm) phases are coupled together to produce a laminated structurewith improved mechanical properties. Scanning electron microscopy (SEM)and transmission electron microscopy (TEM) images of this material areshown in FIG. 1 of this application. Because of this specialarchitecture, composites such as nacre are simultaneously hard, strong,and tough. The core of the organic template is composed of a layer ofβ-chitin layered between “silk-like” glycine-and alanine-rich proteins.The outer surfaces of the template are coated with hydrophilic acidicmacromolecules rich in aspartic and glutamic acids. Recent studiessuggest that these acidic macromolecules alone are responsible forcontrol of the polymorphic form and the morphology of the CaCO₃ (calciteversus aragonite) crystals, although the role of the β-chitin supportedmatrix on the lamellar morphology of the CaCO₃ layers over macroscopicdimensions still remains to be determined.

Morphological and crystallographic analyses of the aragonitic thinlayers of nacre by electron microdiffraction show that c-axis-orientedaragonite platelets form a hierarchical tiling of a twin-related densefilm with twin domains extending over three length scales. Superpositionof the aragonite lattices on all three possible sets of twins generatesa new superlattice structure, which suggests that the organic templateadopts a single-crystalline psuedohexagonal structure. Although cellularactivities leading to the self-assembly or the organic template remainto be understood, the presence of organized organic template isessential to the assembly of the inorganic layer.

In recent years, a number of researchers have demonstrated the viabilityof this approach for the preferential growth of inorganic crystals atthe solid/liquid and liquid/air interfaces. Furthermore, throughchemical modification of these interfaces, by adsorbing surfactants orother reactive moieties, the crystal phase, morphology, growth habit,and even chirality of heterogeneously deposited inorganics can becontrolled.

Mann et al., Nature 332, 119 (1988), describes phase-specific, orientedcalcite crystals grown underneath a compressed surfactant monolayer atthe air/water interface. Changing surfactant type or degree of monolayercompression results in different crystal phases and orientations.

Pacific Northwest National Laboratories (PNNL), B. C. Bunker et al.,Science 264, 48 (1994), describes chemically modifying solid metal,plastic, and oxide surfaces, and the selection of phase and orientationof the depositing crystalline inorganic at a variety of solid/liquidinterfaces. Bunker et al. describes the use of a self-assembledmonolayer (SAM) approach to coat metal and oxide substrates withsurfactant monolayers of tailored hydrophilicity. This is accomplishedby pretreating the substrates with a solution of functionalizedsurfactants, such as sulfonic acid-terminated octadecyl tricholorsilane,before precipitation of the inorganic phase. The choice of theterminating moiety on the surfactant tail determines surface charge andrelative hydrophobicity of the chemisorbed surfactant monolayer. In thisway, oxide and metal substrates can be modified to have the requiredsurface properties to promote inorganic film growth.

A. Kumar and G. M. Whitesides, Appl. Phys. Lett. 63, 2002 (1993); and A.Kumar, H. A. Biebuyck, G. M. Whitesides, Langmuir 10, 1498 (1994),describe a microcontact printing method by which complex, designed SAMpatterns may be transferred onto substrates with an elastomeric stamp.This approach sets up lateral variations in the γ_(is)−γ_(sl) valuealong the substrate and may be used to selectively nucleate and growinorganic phase on the functionalized regions.

B. J. Tarasevich, P. C. Rieke, J. Lin, Chem. Mater. 8, 292 (1996); andP. C. Rieke et al., Langmuir 10,619 (1994), describe the spatiallyresolved deposition of FeOOH mineral through an analogous SAM approachby using electron and ion beam lithography to pattern the SAM layer.This technique allows micrometer-scaled patterning of inorganicmaterials on a variety of substrates through confined nucleation andgrowth of inorganic films.

M. R. De Guire et al., SPIE Proc., in press; and R. J. Collins, H. Slnn,M. R. De Guire, C. N. Sukenik, A. H. Heuer (unpublished) describe theuse of photolithography to pattern the SAM layer prior to area-selectivemineralization of TiO₂, ZrO₂, SiO₂, or Y₂O₃ films.

Kim et al., Nature 376,581 (1995) describes an alternative to the SAMapproach of micromolding in capillaries (MIMIC). In this process,submicrometer-scale patterning of inorganic films is achieved by placingan elastomeric stamp, containing relief features on its surface, intocontact with a substrate. Contact between the elastomeric stamp and thesubstrate forms a network of interconnected channels that may be filledwith an inorganic precursor fluid [such as poly(ethoxymethylsiloxane)]through capillary action. After the material in the fluid iscross-linked, crystallized, or deposited onto the substrate, theelastomeric stamp is removed to leave behind a patterned inorganic filmwith micro-structures complementary to those present in the mold.

S. Manne et al. Langmiur 10, 4409 (1994) and H. Gaub, Science 270, 1480(1995), have shown that three-dimensional surfactant structures such ascylindrical tubules and spheres can be formed at solid/liquidinterfaces. Adsorbed hemi-micellar arrangements were observed on poorlyorienting amorphous substrates, such as silica, and aligned tubularstructures were observed on more strongly orienting crystallinesubstrates such as mica and graphite. The latter substrates orientadsorbed surfactants through anisotropic attraction (either van derWaals or electrostatic) between the crystalline substrate and thesurfactant molecule. The amorphous silica substrate has no preferentialorientation for surfactant adsorption.

Aksay et al., Science 273, 892 (1996) describes a method for theformation of continuous mesoscopic silicate films at the interfacebetween liquids and various substrates. The technique used thesupramolecular assembly of surfactant molecules at interfaces totemplate the condensation of an inorganic silica lattice. In thismanner, continuous mesostructured silica films can be grown on manysubstrates, with the corresponding porous nanostructure determined bythe specifics of the substrate surfactant interaction. XRD analysisrevealed epitaxial alignment of the adsorbed surfactant layer withcrystalline mica and graphite substrates, and significant strain in themesophases silica overlayer. As the films grew thicker, accumulatedstrain was released resulting in the growth of hierarchical structuresfrom the ordered film. This method was used to form “nanotubules” withdimensions of ˜3 nm. Polymerization of the inorganic matrix around thesetubules leads to a hexagonally packed array of surfactant channels.

The aforedescribed techniques represent advances in the selectivenucleation growth of inorganic crystals with specific phase,orientation, and micropatterns. A significant advantage of thebiomimetic processing methods described above is the relatively lowprocessing temperatures involved (typically<100° C.) and the use ofwater rather than organic solvents. Both of these factors render suchmethods relatively environmentally benign. Although continuous films ofthese silicate materials can be formed, the orientation of the tubulesdepends primarily on the nature of the substrate-surfactant interactionand is difficult to control. Once films grow away from the orderinginfluence of the interface, chaotic, hierarchical structures arise.Additionally, there is no facility for organic material to adsorb ontoor to become incorporated within the growing inorganic structure or todo both.

There is thus a need for the development of low-cost lithographictechniques having the ability to pattern “designed” structural featureson the nanometer size scale. Such techniques are important in themanufacture of electronic, opto-electronic and magnetic devices withnanometer scaled dimensions. Technologies involving scanning electronbeam, x-ray lithography and scanning proximal probe are currently underdevelopment, but the practicality of these techniques remains uncertain.Although these continuous films hold much promise for a multitude oftechnological applications (e.g., oriented nanowires, sensor/actuatorarrays, and optoelectronic devices), a method of orienting thenanotubules into designed arrangements is clearly required for thisapproach to become viable as a nanolithographic tool. What is desiredand has not yet been developed is a method that allows the direction ofgrowth of these tubules to be guided to form highly aligned, designednanostructures. It would be desirable that the method is independent ofthe substrate-surfactant interaction and thus allows oriented structuresto be formed on any (non-conducting) substrate.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide a practical, low-costlithographic process that has the ability to pattern “designed”structural features on the nanometer size scale.

It is an additional object of this invention to provide a process thatis useful in the manufacture of electronic, opto-electronic and magneticdevices with nanometer scaled dimensions.

It is a further object of this invention to provide a nanolithographicprocess that orients the nanotubules into designed arrangements.

It is still another object of this invention to provide ananolithographic process that allows the direction of growth of thesetubules to be guided to form highly aligned, designed nano structures.

It is still another object of this invention to provide ananolithographic process that is independent of the substrate-surfactantinteraction and thus allows oriented structures to be formed on any(non-conducting) substrate.

All of the foregoing objects are achieved by the process of thisinvention. The process is directed to preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns. The process comprises:

a) forming a polycrystalline inorganic substrate having a flat surface;

b) placing in contact with the flat surface of the substrate a surfacehaving a predetermined microscopic pattern;

c) placing in contact with an edge of the surface having thepredetermined microscopic pattern, an acidified aqueous reactingsolution, the solution wicking into the microscopic pattern by capillaryaction, wherein the reacting solution comprises an effective amount of asilica source and an effective amount of a surfactant to produce amesoscopic silica film upon contact of the reacting solution with theflat surface of the polycrystalline inorganic substrate and absorptionof the surfactant into the surface; and

d) applying an electric field tangentially directed to the surfacewithin the microscopic pattern, the electric field being sufficient tocause electro-osmotic fluid motion and enhanced rates of fossilizationby localized Joule heating.

BRIEF DESCRIPTION OF THE DRAWINGS

Other important objects and features of the invention will be apparentfrom the following Detailed Description of the Invention taken inconnection with that accompanying drawings in which:

FIGS. 1-7 are taken from Aksay et al., Science 273, 892 (1996) whichdescribes the production of mesoscopic films without the requiredelectric field of this invention. These figures are incorporated in thisapplication for background and comparison.

FIG. 1 (A) A scanning electron microscopy (SEM) (Phillips XL30FEG) imageof fracture surface of aragonitic portion of abalone nacre showingaragonite (CaCO₃) platelets of ˜0.5 μm thick.

FIG. 1 (B) A (TEM) Transmission electron microscopy (Phillips CM200)image of the nacre cross section revealing a <10 nm thin organic film(marked “O”) between the aragonite platelets with their c-axis normal tothe organic template.

FIG. 2 shows SEM images of mesoscopic silica films grown at (A)mica/water, (B) graphite/water, and (C) silica/water interface for 24hours, respectively. Oriented tapes are observed on mica and graphite.The films grown at the silica/water interface are uniform initially(dark background) but spiral-like structures (light features) formlater.

FIG. 3 shows in situ AFM images of mesostructured films growing on mica,graphite, and amorphous silica substrates, respectively. AFM images ofthe mica, graphite, and silica substrates used to grow mesoscopic silicafilms are shown in the insets. (A) and (B) illustrate the periodic micaand graphite atomic lattices, respectively, onto which CTAC adsorbs andorients; (C) reveals a smooth, amorphous silica substrate. Images of thefilms were obtained in “noncontact” mode, utilizing the electricaldouble layer force. (A) Meandering surfactant tubules on the micasubstrate, 6.2 to 6.8 nm spacing, oriented parallel to the solid/liquidinterface. Tubules are initially aligned along one of the threenext-nearest-neighbor directions of the mica oxygen lattice displayed ininset. In the early stages of the reaction (<7 hours), this orientationis preserved as tubules continue to assemble and grow away from theinterface coupled with silica polymerization. (B) On graphite, tubulesalign parallel to the substrate along one of three symmetry axes of thehexagonal carbon lattice shown in the inset. Unlike the structures onmica, these do not meander but form rigid parallel stripes. (C) Onamorphous silica, periodic dimples are observed rather than stripes,suggesting an orientation of the tubules away from the interface.

FIG. 4 shows TEM images of a mesostructured silica film grown on mica.Both images are in a transverse orientation with respect to the film andreveal hexagonal packing of tubules aligned parallel to the substrate.The image in (A) reveals a slight elliptical distortion of the tubulessuggesting that the films are strained, that is, compressed in thedirection normal to the template.

FIG. 5 (A) is a schematic illustration of the sequential mechanism oftemplated, supramolecular surfactant self-assembly on the mica surface(left), followed by intercalation and polymerization of inorganicmonomer to form a mesostructured composite (right). Assembly of thefirst surfactant layer forms a template that defines the structure ofthe subsequent film. On mica, electrostatic interactions between thesubstrate and surfactant lead to complete cylinders that meander acrossthe surface with a loose registry to the underlying substrate lattice.

FIG. 5 (B) is a schematic of a mesostructured silica on graphite. Therigid half-cylinder geometry on graphite occurs because attractive(hydrophobic and van der Waals) interactions between the graphitesurface and the surfactant tails cause them to adsorb horizontally.

FIG. 6 (A) shows a SEM image of a hierarchically structured mesoscopicsilica film grown on a silica substrate. Although all of the filmsappear uniform at early stages of the reaction, once film thicknessesexceed ˜0.5 μm, the ordering influence of the substrate becomes nolonger important. Release of accumulated stain energy within the filmleads to hierarchical structures, with tubule bundles wrapping aroundeach other in three dimensions on several length scales.

FIG. 6 (B) shows a TEM image of a planar cross section of a film grownon silica. The cross section was taken through a macroscopic swirl inthe film shown in (A) and reveals a spiraling and twisting arrangementof surfactant tubules.

FIG. 7 shows a grazing angle of incidence XRD data for mesostructuresilica film growing at the mica/aqueous solution phase interface (after15 hours of reaction time) showing radial scans of two Bragg peaks, the(002) (filled circles) and the (101) (open squares, expanded by a factorof 1350). Growth of the surfactant film on a freshly cleaved micasubstrate results in a highly aligned crystalline lattice, in which the(002) Bragg peak is oriented along the substrate surface normal, with amosaic width that is less than 0.06°. Furthermore, the (101) Bragg peakis also azimuthally aligned within the surface plane, such that thetubules are oriented along the next-nearest-neighbor direction of thesurface oxygen lattice, and having an in-plane mosaic width of ˜10°.Both of these observations clearly suggest that the substrate has astrong orienting effect on the co-assembled film. Further evidence ofthe interaction between the substrate and the co-assembled film can befound in the exact Bragg peak positions. Although bulk mesoscopic silicaexhibits a hexagonal lattice, in which case Q ₀₀₂=Q ₁₀₁, for films grownon miica the radial peak position of the (002) and (101) Bragg peaks arenot equal (Q ₀₀₂=0.139 Å⁻¹ and Q ₁₀₁=0.143 Å⁻¹), which implies that thefilm is strained. From these data, we derive nearest-neighbor spacingsof 52 and 50 Å. This strain results in a ratio of lattice spacings ofb/a=(b/{square root over (3)}a)−1=3.7%, and an area per tube in thecomposite film of 2288 Å² (the inset defines the parameters a and b, andthe lines show the model fit). Strain in these films results fromepitaxial mismatch between the first adsorbed surfactant layer and theperiodic atomic lattice of the substrate.

FIG. 8 shows a scanning electron microscope (SEM) image of an“unconfined” mesoscopic silica film grown on an amorphous silicasubstrate. Once the film becomes thicker than ˜0.5 μm, a chaotic,hierarchical structure of winding tubules is formed.

FIG. 9 is a schematic illustration of the process of this invention usedto induce guided growth of mesoscopic silicate structures.

FIGS. 10 (a) and (b) show SEM images of 1 μm line and square mesoscopicsilicate patterns formed by guided growth within microcapillaries inaccordance with this invention. Electro-osmotic flow is used totransport reacting fluid through the capillaries, and localized Jouleheating triggers rapid polymerization of the inorganic around alignedsurfactant tubules.

FIGS. 11 (a) and (b) show TEM images of a patterned mesoscopic silicastructure grown on a Thermanox plastic substrate (Electron MicroscopySciences) in accordance with this invention. These display a hexagonallypacked surfactant tubule structure within the micron-sized lines shownin FIG. 10. The cross-sectional view of each line reveals an identicalhexagonally packed pattern of tubules, suggesting global alignment oftubules parallel to the substrate and capillary walls. Similar imageshave also been obtained for “confined” films grown on silica substrates.The insert in this FIG. 11 displays the corresponding electrondiffraction

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a process of preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns. The synthesis of silica-basedmesostructured materials by using supramoleclar assemblies of surfactantmolecules to template the condensation of inorganic species is abiomimetic approach to the fabrication of organic/inorganicnanocomposites. This technique holds great promise as a synthetic schemeto produce nanostructured materials with novel properties. For any ofthese applications to be realized, however, what is required is a methodby which these nanostructures can be formed into controlled shapes andpatterns rather than the microscopic particulates that have beenpreviously reported.

The process of this invention includes forming a polycrystallineinorganic substrate having a flat surface and placing in contact withthe flat surface of the substrate a surface having a predeterminedmicroscopic pattern. A network of patterned capillaries is formed byplacing an elastomeric stamp (typically made of polydimethyl siloxane,PDMS) possessing designed relief features on its surface in contact witha flat substrate (see FIG. 9).

Preferably, an ordered silicate structure is used within a highlyconfined space, using the Micromolding in Capillaries (BLOC) technique.Other polycrystalline inorganic substrates may also be used and includea wide variety of transition metal oxides, cadmium sulfide and selenidesemiconductors.

A preferred approach is to start with a well-defined interface such asmica. Under acidic conditions, reactive SiOH anchoring sites on micaprovide binding sites for the silica-surfactant micellar precursorspecies and orient a hexagonal phase of mesostructured silica as acontinuous thin film. It has been found that this approach is not justlimited to the hydrophilic surface of mica but can be generalized toform continuous mesostructured silicate films onto a wide variety ofsubstrates, including hydrophobic surfaces such as graphite. Of primaryconcern is the structure of the first layer of adsorbed surfactant ateach of these interfaces. Although the molecular organization andself-assembly of surfactants at interfaces is a widely studied area,little is still known about the precise structure of adsorbed surfactantlayers.

An acidified aqueous reacting solution is then placed in contact with anedge of the surface having the predetermined microscopic pattern. Thesolution wicks into the microscopic pattern by capillary action. Thereacting solution has an effective amount of a silica source and aneffective amount of a surfactant to produce a mesoscopic silica filmupon contact of the reacting solution with the flat surface of thepolycrystalline inorganic substrate and absorption of the surfactantinto the surface.

It is important for the biomimetic processing of thin inorganic films tomaintain relatively low levels of supersaturation during theprecipitation process in order to minimize the amount of particleformation in bulk solution.

All mineralization processes involve the precipitation of inorganicmaterial from solution. A key requirement for successful film formationis to promote the formation of the inorganic phase on the substratedirectly (that is, heterogeneous nucleation) and prevent the homogeneousnucleation of particles in the solution. According to classicalnucleation theory, the free energy change (ΔF) associated with theprecipitation of an inorganic cluster from solution onto a surface isgiven by:

ΔF=−nk _(B) Tln S+γ _(il) A _(il)+(γ_(is)−γ_(sl))A _(is)  (1)

where S represents the degree of supersaturation in the fluid; n is theaggregation number; k_(B) is Boltzmann's constant; T is temperature;γ_(il), γ_(is), and γ_(sl) represent the inorganic/liquid interfacialtension, respectively; and A_(il) and A_(is) represent the correspondinginterfacial areas.

When the interaction between the growing nucleus and substrate surfacerepresents a lower net interfacial energy than the inorganic/solutioninterfacial energy i.e., (γ_(is)−γ_(sl))A_(is) <γ_(il)A_(il),heterogeneous nucleation is favored over homogeneous nucleation. This isthe case for the majority of precipitating inorganic systems, and henceheterogeneous nucleation is the dominant precipitation mechanism forthermodynamically controlled systems. Homogeneous nucleation will onlydominate at relatively high levels of supersaturation where theprecipitation process becomes kinetically controlled. Precipitationtimes for homogeneous nucleation vary enormously, from months tomilliseconds, depending sensitively on the value of S.

More specifically, in order to promote growth of a mesostructuredinorganic on these substrates, an aqueous recipe that includes an excessof adsorbing cetyltrimethyl ammonium chloride (CTAC) surfactant and adilute acidic solution of tetraethoxy silane (TEOS) inorganic precursoris used. Inorganic solute concentrations are purposefully kept dilute inorder to decrease the rate of homogeneous nucleation to such an extentthat the more thermodynamically favored heterogeneous nucleation routeis dominant. The procedure involved dissolving TEOS liquid in an aqueoussolution of CTAC and hydrochloric acid. Typical molar ratios are 1TEOS:2 CTAC:9.2 HCl:1000 H₂0.

The formation of a mesoscopic silica film begins to occur immediatelyupon contact of this solution with any interface onto which thesurfactant can adsorb. A dilute solution of the TEOS silica source isspecifically used to prevent homogeneous nucleation of inorganicmaterial in bulk solution, and to promote heterogeneous nucleation andgrowth of a mesoscopic film at the substrate/solution interface.

Aksay et al, Science 273, 892 (1996), the entire disclosure of which isincorporated herein by reference, describes the production of mesoscopicfilms without the required electric field of this invention. FIGS. 1-7are taken from this reference for background and comparison.

FIG. 2 shows SEM images of mesoscopic films grown for a period of 24hours at the mica, graphite, and silica/water interfaces. Under similarconditions, freestanding mesostructured silica films can also be grownat the air/water interfaces. All of the films are continuous and displaydistinctly different textures at length scales between 0.5 and 10 μm.

FIG. 3 shows in situ AFM images of the atomic lattice of each substrateas well as the structure of the mesoscopic silica overlayer growing oneach surface. To obtain these images, a method was used that useselectrical double layer repulsive forces to image the chargedistribution of an adsorbed layer on the sample. The images of the outerlayer of the reacting mesostructured film were obtained by immersing theimaging tip and substrate in the reacting mixture and, once sufficienttime was allowed for thermal and mechanical equilibration, setting theimage setpoint in the repulsive precontact region. In this way, the tipis held ˜1 nm above the reacting surface and the scanning motion of theAFM produces a topological map of charge density.

In the case of mica, FIG. 3A reveals meandering stripes with a spacingof 6.2 to 6.8 nm.

These are observed at every stage of the reaction. After 10 hours ofreaction, in situ AFM images are difficult to obtain because of theappreciable growth of mesostructured silica on the top surface of theAFM flow cell and cantilever spring. The presence and irregular natureof both of these films disturb the reflection of the laser light beamused to monitor spring deflection. As discussed below, x-ray diffraction(XRD) analysis of these films reveals a distorted hexagonal stacking ofsurfactant tubules (5.6 nm nearest-neighbor spacing) that lie parallelto the surface and are axially aligned along the next-nearest-neighbordirection of the hexagonal oxygen lattice on the mica surface.

FIG. 4 shows TEM images of a mesostructured film on mica, cut in twodifferent transverse directions. All three methods reveal a consistentstructure of the mesostructured film on mica. AFM images similar tothose in FIG. 3A were obtained without TEOS present, but theseinterfacial surfactant films are limited to one or two layers ofcylindrical tubules. AFM studies on systems containing only surfactant,with no TEOS, reveal the presence of several layers of adsorbedsurfactant tubules. Three-dimensional “multilayer” features have beenimaged with the microscope, and as many as three “steplike” features areobserved in the repulsive portion of the force-distance curve near thesubstrate (M. Trau et al., in preparation). The existence of suchsupramolecular surfactant structures in the absence of TEOS suggests asequential reaction mechanism involving surfactant self-assemblyfollowed by inorganic condensation.

The self-assembly of micellar layers without the presence of theinorganic agent suggests a sequential growth and polymerization for thesilicate films (FIG. 5A). First, the surfactant self-assembles on themica substrate to form meandering tubules, and second, silicon hydroxidemonomers (or multimers) polymerize at the micellar surface. Aspolymerization continues, more surfactant is adsorbed to the freshlyformed inorganic surface and allows the templated mesoscopic structureto replicate itself and grow in to the bulk solution. After growthperiods of 24 hours, the mesoscopic composite films begin to developlarger scale structural features such as those shown in FIG. 2A. At thisstage, aligned “tapes” and steps appear with macroscopic grain boundaryangles 60° and 120°. These macroscopic angles clearly result from atomiclevel registry of the surfactant tubules with the underlying micalattice.

For graphite substrates (FIGS. 2B and 3B), the surfactant tubules arealso aligned parallel to the surface, but in this case they are rigid,parallel stripes without the meandering curvature observed on mica.Measured nearest-neighbor spacings similar to that seen on mica andmicroscopic grain boundaries can be clearly imaged, which again suggestsa preferential axial orientation of the surfactant tubules with thehexagonal graphite lattice. The graphite surface is distinct from micain that it is hydrophobic and does not contain ionizable moieties toengender surface charge. Attractive interactions hydrophobic and van derWaals) between the graphite surface and surfactant tails cause them toadsorb horizontally (FIG. 5B), and the resulting large interaction areaper molecule gives rise to a strong orientation effect between moleculeand substrate that is preserved in the cylindrical aggregates. Micainteracts only with the head group and orients the adsorbed moleculesvertically; the smaller interaction area gives rise to a correspondinglysmaller orientation effect. At long reaction times, macroscopic featuresgrow out of the oriented, uniform film similar with macroscopic anglesof 60° and 120° are also observed.

Growth of these films at the silica/water interface gives rise to silicafilms with macro and microstructures dramatically different from theones described above. FIG. 3C shows an in situ AFM image of the reactingfilm grown from a silica substrate. Rather than the parallel stripesobserved on the previous substrates, this image shows periodic arrays ofdimples suggesting an orientation of surfactant tubules out of the planeof the interface. An XED analysis confirms a distorted hexagonal packingof the tubules. The dimpled pattern suggests a twisting arrangement ofhexagonally packed tubules attached to the interface at one end andspiraling into the solution. Similar dimpled structures were alsoobserved with neat CTAC solution, which suggests the formation ofroughly spherical surfactant aggregates that act as starting points onthe surface for growth of cylindrical tubules into the solution.Micellar structures of quaternary ammonium surfactants on silica havebeen previously postulated and observed.

As in the case of mica and graphite, the structure formed in the silicasubstrate films is a direct consequence of the arrangement of the firstlayer of adsorbed surfactant on the surface. It appears that theordering ability of the silica interface, which is dramaticallydifferent from that of mica and graphite, is not great enough to confinethe surfactant tubules to lie straight on planar surfaces. Indeed,having nucleated one end of the tubules at the interface, the long axesof the tubules appear to wander over a wide range of slowly curvingconfigurations in three dimensions, suggesting that it takes very littleenergy to bend the tubules along their long axes. This effect may simplybe understood in terms of a Helfrich (W. Z. Helfrich, Natur for Chung28C, 693 (1973)) bending energy model of the tubule surfactant layer:$\begin{matrix}{E = {{\frac{k_{c}}{2}\left( {\frac{1}{R_{1}} + \frac{1}{R_{2}} - \frac{1}{R_{0}}} \right)^{2}} + \frac{k_{g}}{R_{1}R_{2}}}} & (2)\end{matrix}$

where E is the free energy per unit area (effectively, an energy persurfactant molecule); k_(c) and k_(g) are the rigidity and Gaussiancurvature constants, respectively; and R₁, R₂, and R₀ are the principleradii and the spontaneous radius of curvature, respectively. Althoughthis form was derived for the thin-film limit in which the radii arelarge compared to the thickness of the surfactant layer, it also appearsto describe reasonably well certain cases in which the surfactant layerthickness is comparable to R₁ and R₀. R₁, the small radius of thetubule, is strongly constrained by the length of the surfactantmolecules. Insofar as R₀ is fixed by the surfactant composition and issmall (˜5 nm), as is typically the case for single-chain surfactants,and if R₂>>R₁ as is the case for long, thin tubules, then Eq. 2 is wellapproximated by: $\begin{matrix}{E = {{k_{c}/2}\left( {{1/R_{1}} - \frac{1}{R_{0}}} \right)^{2}}} & (3)\end{matrix}$

In other words, the energy of bending along the long axis of a tubuledoes not figure prominently into the bending energy. Unless order isimposed on the tubules by external forces, such as adsorption forces,the tubules will sample a wide range of slowly varying configurations.This prediction is also consistent with observed macroscopic structuresof mesoscopic silica films formed after long growth times at thesilica/water interface (FIG. 6). These films begin growing as veryuniform structures but soon become increasingly textured and chaotic asthe film thickness increases. Rather than the oriented tapes observed inthe cases of mica and graphite substrates, the silica substrate filmsdisplay chaotic, spiral-like structures wrapped in a hierarchicalfashion around each other (FIGS. 2C, 3C, and 6). The films becomeincreasingly disordered once the thickness is great enough such that thesurface can no longer induce ordering.

Aksay et al., investigated the substrate ordering effect on surfactanttubules and subsequent mesoscopic silica films, by performing XRDanalyses of films grown on mica. In these measurements, the growth wasterminated by partially draining the solution in a sealed cell andperforming the measurements while the sample was in contact with thevapor of the growth solution. FIG. 7 indicates that the films arestrained in the plane perpendicular to the substrate, with the hexagonalpacking of tubules distorted by as much as 4% during growth. Upondrying, strain within the film is significantly altered. It was observedby Aksay et al. that there was +18% strain in the film grown in thenonequilibrium condition, in the case where the solution is confined ina thin film geometry by a wrap. It was also found that the film growsabout an order of magnitude faster in this condition. There was a largechange in the Bragg peak position as these films dry that corresponds tolarge changes in the film lattice constant and strain. For example, thedegree of hexagonal strain varies from +18% while wet to +2% a few hoursafter removal from solution, and finally −7% several days later whenthey are dry. In this process, the lateral spacings of the filmincreased by 2% upon drying and finally by 5% after a few days. Thisimplies that the 25% change in a/b ratio of the film upon drying waslargely due to a change in the vertical lattice spacings due to dryingshrinkage in the normal direction.

During growth, the strain appears to result from the ordering influencethat the mica substrate exerts on the adsorbed surfactant tubules. Thatis, the forces that act to align the tubules parallel to the surfacealso act to deform the hexagonal packing in three dimensions. The forcesresponsible could be either van der Waals or electrostatic in naturebecause the mica surface has ionizable moieties. As the self-assembledorganic layers grow away from the surface, the ordering effect isexpected to diminish. Experiments performed in the absence of the TEOSinorganic precursor revealed that one or two layers of surfactanttubules can adsorb to the substrate prior to silica condensation. Oncethe TEOS is included in the solution, silica begins to condense withinthe adsorbed surfactant layers and films grow away from the surface.More layers of surfactant can now adsorb to the freshly formed silicainterface, which provides a mechanism for the film to continue to growout into the solution. This growth mechanism, however, does not relievethe original strain in the film. Moreover, as the film grows thicker,tubules adsorbed to the mesostructured silica will be staineddifferently to the initial layers adsorbed on the mica surface. Evidencefor the eventual release of this strain is seen most clearly on thesurface of films grown for long periods. An example is seen in FIG. 2A,where macroscopic features such as the “swirling tube” and “hook” appearand grow out of the aligned film in wormlike manner. On mica, thesefeatures begin to occur at film thicknesses of ˜0.5 μm and alwayspossess a wormlike structure. For films grown at the silica/waterinterface, dramatically different structures are seen to grow out of thefilm at similar film thicknesses (FIGS. 2C and 6). Although the firstlayer of tubule structure is different for each substrate, the releaseof accumulated strain within these films through the growth of tubulebundles away from the oriented film is a common feature of all of thesefilms. For mica and graphite, these bundles form wormlike structures,and for silica, tapes and spirals are formed that wrap around each otherin a hierarchical manner. The hierarchical structures formed in thickfilms thus appear to result from the release of accumulated strainenergy associated with the epitaxial mismatch between the first layer ofadsorbed surfactant and the periodic atomic lattice of the substrate. Inall cases, this is observed to occur only for relatively thick films(≧0.5 μm) where the ordering influence of the substrate no longerexists.

The invention herein requires that after the acidified aqueous reactingsolution be placed in contact with an edge of the surface having thepredetermined microscopic pattern, and that an electric field be appliedand directed tangentially to the surface within the microscopic pattern.The electric field should be sufficient to cause electro-osmotic fluidmotion and enhance the rates of fossilization by localized Jouleheating.

It has been found that within the capillaries, because the reactingsolution is dilute, reactants are quickly depleted, and film growthceases. Moreover, the growth of mesoscopic film at the edges of the moldseals the capillaries and prevents diffusion of reacting species to theinterior of the mold. To maintain a uniform concentration of reactantswithin the capillaries during the growth process, the electric field isapplied parallel to the substrate in the manner illustrated in FIG. 9.

Application of an electric field in this geometry has three effects: itinduces elctro-osmotic fluid flow; it aligns surfactant tubules; and itcauses localized Joule heating of the solution. These effects aresynergistic in guiding and fossilizing the silicate mesostructureswithin the microcapillary reaction chambers. For applied fields >0.1 kVnm⁻¹, electro-osmotic fluid flow is observed within the capillaries, asa result of the interaction of the field with the ionic double layercharge near the capillary wall. Surface charge on the capillary wallsarises from adsorption of the positively charged CTAC surfactant.Maintaining a steady fluid flow through the capillaries during theentire growth process ensures that the reactant concentration withineach micro reaction chamber remains constant with time—this constancyallows uniform films to be grown.

FIG. 10 shows SEM images of square and lined patterns of mesoscopicsilica grown on a silica substrate after 5 hours of reaction time. A DCfield of 0.15 kV mm⁻¹ was applied during the entire reaction process,and fresh reacting fluid was continuously dripped on one side of themold to replenish the volume removed by the electro-osmotic flow. Ineach case, the patterns formed replicate the structures of the mold.Within the capillaries, films begin to grow on all exposed surfaces,i.e., at both the PDMS mold and the substrate/aqueous solutioninterface. As the reaction progressed, the capillaries narrowed in thecenter and eventually sealed completely.

The high conductivity of the acidic reaction solution gives rise tosignificant Joule heating at these applied voltages. Positioning theelectrodes in an excess reservoir of reacting solution outside themicrocapillary volume (FIG. 9) allows high fields to be applied acrossthe aqueous solution confined within the capillaries. In such a scheme,rapid electrolysis (H₂O→H₂+½O₂) ensues, however bubble formation isconfined to the fluid reservoir near each electrode and does not disturbthe formation of silica mesostructures within the capillaries. Atvoltages of 1 kV mm⁻¹, sparks are occasionally observed within the fluidconfined in the microcapillary as a result of intense localized heating.At lower fields, sparks are not observed and the Joule heatingaccelerates the fossilization rate of the mesoscopic silica byincreasing the rate of polymerization of TEOS precursor to silica.

With no applied field, 0.5 μm thick films are typically grown in aperiod of 24 h; with an applied field of 0.1 kV mm⁻¹, similarthicknesses are achieved in 1-5 h. Localized heating of the reactingsolution in this manner provides a useful method of rapidly rigidifyingthe aligned surfactant tubular structures formed within themicrocapillaries.

In order to determine the orientation of the surfactant nanotubuleswithin these structures, cross-sectional samples were prepared using aLeica ultramicrotome and analyzed by high resolution transmissionelectron microscopy (TEM). FIG. 11 shows a typical example of theresulting TEM images as well as a typical selected area electrondiffraction pattern (SAED). These reveal a hexagonally packedarrangement of tubules with a nearest neighbor spacing of 3.0 nm.Detailed examination of diffraction pattern reveals a slightly distortedpacking arrangement, with a deviation of 4% from perfect hexagonal. Thisdistortion may be a result of the accelerated fossilization processdescribed above: with no applied field, no distortion is observed.

Multiple cross-sections taken of the 1 μm line structures shown in FIG.10 all appear identical to the image shown in FIG. 11. This indicatesthat all tubules within the capillaries are aligned parallel to thesubstrate, the long axis of the capillary, and the direction of theapplied field. This results in a dramatic contrast to the “unconfined”mesoscopic silica film synthesis, which always results in a chaotic andnon-aligned arrangement of tubules (FIG. 8). In the process of thisinvention, rather than taking on a random configuration, growing tubulesare guided within the confined space of the capillary and remainparallel to the walls. This orientation occurs either as a result of theaction of the external field, i.e., alignment of tubules resulting frompolarization body forces that operate in regions of dielectric constantgradient (˜∇εE²) or by virtue of the confined space within which thereaction is performed. In both cases, the tubules would be alignedparallel to the capillary walls. For field-induced alignment, suchconfigurations minimize the overall electrostatic energy—provided adifference in dielectric constant exists between the inner and outervolume of the tubule. It is also known that the formation of end-caps inself-assembled surfactant cylinders is not favored, given their highfree energy of formation. Thus, within, a highly confined region,surfactant cylinders will take on configurations which minimize thenumber of end-caps. Consequently, they will tend to elongate along thelong axis of the capillary rather than truncating at capillary walls.

In the absence of an ordering field, a wide range of slowly curvingconfigurations of tubules if formed in three-dimensions (FIG. 8). Asdescribed previously, such a configuration can be understood in terms ofa simplified Helfrich, W. Z., Naturforch. 28C, 693 (1973), bendingenergy model of the surfactant tubule, E=K_(c)/2(1/R₁−1/R₀)², where E isthe free energy per unit area (effectively, an energy per surfactantmolecule in the tubule), k_(c) is a rigidity constant, and R₁ and R₂ arerespectively the principal and spontaneous radius of curvature of thetubule. In so far as R₀ is fixed by the surfactant composition and issmall (˜5 nm), as is typically the case for single chain surfactants,the above equation shows complete insensitivity to R₁ for values>>R₀.This analysis implies that the energy of bending along the long axis ofa tubule does not figure prominently in the bending energy. Thus, unlessorder is imposed on the tubules by external forces, such as adsorptionforces, or an electric or flow field, the tubules will sample a widerange of slowly varying configurations. This concurs during “unconfined”film growth, where it was shown that orientation and alignment oftubules can be controlled in the initial stages of film growth bymanipulating the strength and nature of the specificsurfactant-substrate interactions. Although this growth scheme givessome control over the film structure, once the film grows away from theinterface, the orientation that existed in the first layers begins to belost as the ordering influence on the interface diminishes. In our case,the combined influence of confining geometry and applied field allowsthe synthesis of mesoscopic silicate nanostructures with preciselycontrolled geometries. In this way, the tubule geometry is controlled inall regions of the film and the synthesis can be performed on anyrequired substrate, regardless of the nature of the surfactant-substrateinteraction.

An enormous variety of patterns can be formed using the MIMIC approach,with nanotubules aligned parallel to capillary walls. Capillarythicknesses of 1 μm, corresponding to roughly 300 nanotubules, areeasily achieved by this method and thinner structures can also be formedusing molds formed from masters prepared by electron beam lithography.

As a viable method for the production of thin films with complexnanometer and micro-scaled hierarchical architecture, the guided growthof mesoscopic silicates within confined geometries provides a convenientmethod for fabrication of nanostructured materials in a variety ofapplications ranging from sensors and actuators to optoelectronicdevices.

Having thus described the invention in detail, it is to be understoodthat the foregoing description is not intended to limit the spirit andscope thereof. What is desired to be protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A process for preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns, comprising: a) forming a polycrystallineinorganic substrate having a surface; b) placing in contact with thesurface of the substrate, a surface having a predetermined microscopicpattern; c) placing in contact with the surface having the predeterminedmicroscopic pattern, an acidified aqueous reacting solution, wherein thereacting solution comprises an effective amount of a silica source andan effective amiount of a surfactant to produce a mesoscopic silica filmupon contact of the reacting solution with the surface of thepolycrystalline inorganic substrate; d) applying an electric field tothe surface of the microscopic pattern, the electric field beingsufficient to cause enhanced rates of fossilization.
 2. A process forpreparing surfactant-silicate nanostructured materials having designedmicroscopic patterns, comprising: a) forming an ordered silicatestructure substrate having a surface; b) placing in contact with thesurface of the substrate a surface of a stamp, the stamp surface havingrelief features comprising a predetermined microscopic pattern; c)placing in contact with the stamp surface an acidified aqueous reactingsolution, wherein the reacting solution comprises an effective amount ofa silica source and an effective amount of a surfactant to produce amesoscopic silica film upon contact of the reacting solution with thesurface of the ordered silicate structure substrate; d) applying anelectric field to the surface of the microscopic pattern, the electricfield being sufficient to cause enhanced rates of fossilization.
 3. Aprocess for preparing surfactant-silicate nanotubule structures havingoriented patterning, comprising: a) forming an ordered silicatestructure substrate having a surface; b) placing in contact with thesurface of the substrate a surface of a stamp, the stamp surface havingdesign relief features comprising a network of channels; c) placing incontact with the surface of the stamp an acidified aqueous reactingsolution, wherein the reacting solution comprises an effective amount ofa silica source and an effective I_ amount of a surfactant to produce amesoscopic silica fihn upon contact of the reacting solution with thesurface of the ordered silicate structure substrate; d) applying anelectric field to the surface of the stamp, the electric field beingsufficient to cause enhanced rates of fossilization.
 4. A process forpreparing surfactant-silicate nanotubule structures having orientedpatterning, comprising: a) forming an ordered silicate structuresubstrate having a surface; b) placing in contact with the surface ofthe substrate a surface of a stamp, the stamp surface having designrelief features comprising a network of channels; c) placing in contactwith the stamp surface an acidified aqueous reacting solution, whereinthe reacting solution comprises an effective amount of tetraethoxysilane(TEOS) and an effective amount of a surfactant to produce a mesoscopicsilica film upon contact of the reacting solution with the surface ofthe ordered silicate structure substrate; and d) applying an electricfield to the surface within the network of channels, the electric fieldbeing sufficient to cause enhanced rates of fossilization.
 5. Theprocess of claim 4, wherein the effective amount of tetraethoxysilane(TEOS) and the effective amount of the surfactant are sufficientlydilute to prevent homogeneous nucleation of tetraethoxysilane (TEOS)inorganic material in the solution prior to placing the solution incontact with the stamp surface.
 6. The process of claim 2, 3 or 4,further comprising removing the stamp after fossilization.
 7. Theprocess of claim 2, 3 or 4, wherein the surfactant iscetyltrimethylamnmonium chloride (CTAC).
 8. The process of claim 7,wherein the reacting solution has a molar ratio of about 1 TEOS:1.2CTAC:9.2 HCI:1000 H₂P.
 9. The process of claim 3 or 4, wherein the stampcomprises polydimethyl siloxane, (PDMS).
 10. The process of claim 2, 3or 4, wherein the ordered silicate structure is produced within a highlyconfined space using the Micromolding in Capillaries (MIMIC) technique.11. The process of claim 1, 2, 3 or 4, wherein the electric field isabout 0.1-1 kVmm⁻¹.